OP and others...
You can set, via accessibility, the triple-click home button to Invert Colors. Then, when reading some text, you can triple-click the home button and have a black background with white text.
Reading black text on a white background is more fatiguing in any kind of low light, as your eyes are trying to make out dark shapes within a light source (background). Since light tends to bleed into darker areas, this requires more effort as your eyes can't be adjusted for both levels.
Reading white text on a black background is less tiring because the overall brightness is much, much lower, and the light bleeds away from the letters rather than into them, making them easier to distinguish.
This applies to back-lit sources, like screens. It's also why I use ChangeColors in Chrome, which works way better than invert colors anyways.